The present invention relates to a method of producing syngas by converting methane and other hydrocarbons in a gas feed into hydrogen and carbon monoxide. More particularly, the present invention relates to such a method in which the methane is converted in two or more stages to allow an initial catalytic reforming stage formed by an oxygen transport membrane reactor to be operated at a lower temperature than a subsequent stage to protect the structural integrity of the membrane.
The use of oxygen transport membranes in high temperature catalytic reactors for the generation of syngas has received significant attention in the recent past because of the economic incentives created by combining oxygen separation and the syngas generating oxidation and reforming reactions in a single process unit. The process involves bringing a mixture of hydrocarbons, steam, and optionally carbon dioxide in contact with the anode side of an electron and oxygen ion conducting ceramic membrane which at high temperature permeates oxygen from an oxygen containing gas, typically air, on the cathode side to the anode. The preferred membranes consist of mixed conducting metal oxide films supported by multi-layer porous structures which near the membrane film enhance surface exchange and are mechanically and chemically compatible. The permeated oxygen will react with the fuel gas in a partial oxidation reaction, which provides the energy for a simultaneous catalytically enhanced reforming reaction to produce a mixture of hydrogen and carbon monoxide, i.e. syngas.
The partial oxidation reaction for methane is shown in Equation 1. The steam reforming reaction for methane is shown in Equation 2. Additional conversion of carbon monoxide may occur with the exothermic water gas shift reaction, Equation 3. The scope of the present invention also includes reforming reactions between methane and CO2 as indicated by equation (4).
CH4+xc2xdO2xe2x86x92CO+2 H2xe2x80x83xe2x80x83(1) 
CH4+H2Oxe2x86x92CO+3 H2xe2x80x83xe2x80x83(2) 
CO+H2Oxe2x86x92CO2+H2xe2x80x83xe2x80x83(3) 
CH4+CO2xe2x86x922CO+2H2xe2x80x83xe2x80x83(4) 
U.S. Pat. No. 4,793,904 describes an electrocatalytic process for producing synthesis gas from light hydrocarbons. The process involves passing an oxygen containing gas over the cathode side of an oxygen ion conducting membrane and permeating oxygen from the cathode to the anode to react with light hydrocarbons and thus producing a synthesis gas. U.S. Pat. No. 5,714,091 discloses a process for generating syngas using an autothermal reactor comprising an oxygen ion conducting membrane. The process stages a partial oxidation reaction with a catalytic reforming reaction where heat is transferred from the former to the latter.
Typically the syngas is produced at high pressure (10 to 40 bar) to avoid cost associated with compressing the low density syngas. To achieve high conversion of methane at the desired hydrogen to carbon monoxide ratios requires favorable equilibrium conditions for the reactant gases which at the prevailing pressures can be obtained only at relatively high temperatures, e.g. 900 to 1100xc2x0 C. Unfortunately, at these temperatures the preferred membrane materials have low creep strength which leads to limited membrane life and/or complicated structural supports. The problem is further aggravated by the fact that the actual temperatures experienced by the membrane are even higher since a temperature gradient is needed to dissipate the heat of the exothermic oxidation reaction, occurring at or near the anode surface, to the endothermic reforming reaction in the adjacent catalyst structure.
Another problem relates to solid carbon formation, which can be especially acute when the feed stock contains hydrocarbons heavier than methane and it is desired to operate at low steam to carbon ratios. Even with only moderate amounts of hydrocarbons with more than one carbon atom in the natural gas feed, and at the low steam to carbon ratios ( less than 1) desired for processes producing syngas with hydrogen to carbon monoxide ratios of about two or less, permissible inlet temperatures for the reactor, i.e. values above which free carbon formation would occur, would be below the level where appreciable oxygen ion transport rates can be realized. U.S. Pat. No. 6,077,323 addresses syngas generation using mixed conducting ceramic membranes. It discloses operating temperatures for the membrane reactor which avoid solid carbon formation and specifies a higher total pressure on the anode than on the cathode side. Beyond the use of a prereformer the patent does not involve a staged process to optimize operation of the membrane reactor-reformer or limit operating temperatures of the membrane. U.S. Pat. No. 6,048,472 by Nataraj et al. stages a catalytic prereformer, operating at a lower temperature, ahead of the membrane reactor to eliminate carbon formation, especially with higher hydrocarbons, and raise the permissible inlet temperature for the membrane react. It is to be noted that the prior art routinely employs prereformers in conjunction with conventional autothermal or externally fired reformers to eliminate solid carbon formation with heavier feed stocks and or at low steam to carbon ratios. Representative patents illustrating the use of prereformers in conjunction with autothermal and conventional steam reformers are U.S. Pat. Nos. 5,252,609, 4,631,182 and 4,824,658.
As will be discussed, the present invention presents a multi-stage processes for syngas generation by oxygen transport membrane reactors that include prereformers or catalytic partial oxidation reactors for pretreatment of the hydrocarbon feed streams and oxygen transport membrane reactor-reformers coupled with subsequent reforming or autothermal reforming reactors to allow the oxygen transport membrane reactor-reformers to operate at sufficiently low temperatures to address structural problems with preferred oxygen transport membrane materials.
The present invention provides a method of producing a crude syngas product stream or a syngas product stream by further processing of the crude syngas product stream. Both the crude and syngas product stream comprise carbon monoxide and hydrogen. The crude syngas product stream additionally comprises carbon dioxide and moisture. In accordance with the method, methane in a feed stream comprising methane is converted into the hydrogen and carbon monoxide in at least two stages, thereby to form a crude syngas stream. The methane is converted at least in part by carbon dioxide or steam methane reforming in one of the at least two stages operated at a lower temperature than a subsequent of the at least two stages. The one of the at least two stages has at least one oxygen transport membrane to separate oxygen from an oxygen containing gas, thereby permeating oxygen from a cathode to an anode side thereof, and a reforming catalyst located adjacent said anode side of the at least one oxygen transport membrane to promote the reforming of the methane. The reforming of the methane is thermally balanced through heat generated by oxidation of fuel species supported by the oxygen permeating through said at least one oxygen transport membrane. The subsequent of the at least two stages can be a fired reformer or an autothermal reformer, sometimes also called an oxygen blown reformer. In case of the formation of a syngas product stream, water and at least part of the carbon dioxide is removed from said crude syngas stream. In certain downstream processes it is advantageous to leave some of the carbon dioxide in the syngas product stream. Part of said syngas product stream can be recycled to form part of the feed stream as can be off-gases from downstream synthesis reactions.
Preferably, the methane in the feed stream can be converted into the hydrogen and carbon monoxide by partial oxidation of the methane prior to the reforming of the methane within an entrant section of the at least one oxygen transport membrane not containing said reforming catalyst. Higher order hydrocarbons within said feed can be converted into a portion of the methane to be converted into said hydrogen and carbon monoxide prior to the partial oxidation of the methane. Such conversion can take place through catalytic partial oxidation or through steam methane reforming. Since part of the methane content of the feed is so converted to syngas, such prereforming or catalytic partial oxidation is considered a further stage of the two stages mentioned above. Sulfur can be removed from the feed prior to the conversion of said higher order hydrocarbons.
The present invention allows for a solution of the problem posed by mechanical limitations of preferred membrane materials by the use of a two stage process in which an oxygen transport membrane reactor forms a first lower temperature stage and an autothermal reactor or externally fired reformer, operating at a higher temperature, forms a high temperature second stage. In this regard, a preferred operating temperature range of the lower temperature stage formed by the membrane reactor is between about 800xc2x0 C. and about 850xc2x0 C. At such a temperature range and at a desired process gas pressure of between about 7 bar and about 30 bar, prevailing equilibrium conditions lead to the conversion of only about 75 to 90 percent of the hydrocarbons contained in the membrane reactor feed. The remaining hydrocarbons are converted in the second stage that does not use a ceramic membrane and which operates at a higher temperature to produce more favorable equilibrium conditions. Preferably the second stage reactor is an autothermal device consisting of a combustion section and a catalytic reforming section. The overall reaction in the autothermal unit is exothermic which permits raising the outlet temperature from the second stage to the desired level of between about 950xc2x0 C. and about 1100xc2x0 C. Optionally steam and or carbon dioxide can be added to the oxygen feed to adjust the hydrogen to carbon ratio in the synthesis gas product, to control the exothermicity of the process, and to produce more favorable equilibrium conditions. Steam and carbon dioxide can also be added to the feed of the alternate fired reformer stage.
The problem of avoiding solid carbon formation is solved in the present invention by insertion of a catalytic prereforming or partial oxidation step and operating the first section of the oxygen transport membrane reactor as a partial oxidation unit by omitting reforming catalyst in the entrant section thereof. The reaction of the feed with permeated oxygen generates additional hydrogen that contributes to avoiding conditions favorable to solid carbon formation. With respect to carbon formation, preferably a method in accordance with the present invention is conducted such that the inlet temperature of the oxygen transport membrane reactor is not greater than about 750xc2x0 C. and preferably within a range of between about 700xc2x0 C. and about 750xc2x0 C. As has been mentioned above, the prior art addressed the problem of solid carbon formation by inserting a catalytic reformer, operating at temperatures from between about 450xc2x0 C. to about 550xc2x0 C., to convert the heavier hydrocarbons and generate some beneficial hydrogen to suppress carbon formation. A catalytic reformer could be used over the same temperature range in connection with the present invention. The catalytic partial oxidation alternative of the present invention preferably operates in a temperature range of between about 400xc2x0 C. and about 700xc2x0 C. with an oxidant of high purity oxygen to avoid contaminating the product gas with excessive amounts of inerts. Such a partial oxidation step contributes a portion of the preheating requirements for the feed stock.